Elimination of radiation-induced γ-H2AX foci in mammalian nucleus can occur by histone exchange

Double-strand breaks in mammalian DNA lead to rapid phosphorylation of C-terminal serines in histone H2AX (γ-H2AX) and formation of large nuclear γ-H2AX foci. After DNA repair these foci disappear, but molecular mechanism of elimination of γ-H2AX foci remains unclear. H2AX protein can be phosphorylated and dephosphorylated in vitro in the absence of chromatin. Here, we compared global exchange of GFP-H2AX with kinetics of formation and elimination of radiation-induced γ-H2AX foci. Maximal number of γ-H2AX foci is observed one hour after irradiation, when ∼20% of GFP-H2AX is exchanged suggesting that formation of the foci mostly occurs by in situ H2AX phosphorylation. However, slow elimination of γ-H2AX foci is weakly affected by an inhibitor of protein phosphatases calyculin A which is known as an agent suppressing dephosphorylation of γ-H2AX. This indicates that elimination of γ-H2AX foci may be independent of dephosphorylation of H2AX which can occur after its removal from the foci by exchange.